i know you have to connect to black city with the extralink but what if the black city doesn't have marie in it? How do you get her? in other words, how do you get marie into black city so I can get her to the white forest with the extralink? I desperately want a magnemite but no one wants to trade with me.

You need to have a copy of black and white. So if your using White then you need to get a copy of Pokemon black and another console so you can enter the entralink with some-one else. I don't know what to do to get Marie to your White Forest though :/